1. Field of the Invention
The present invention relates to a method for manufacturing a liquid crystal display, and more particularly to a method for manufacturing a high aperture TFD LCD device.
2. Description of the Prior Art
Nowadays liquid crystal display (LCD) devices are widely applied in daily life, such as display devices of PAD, PC, Notebook, TV, electronic watches, digital clocks, operating interface of copy machines, or odometer and speedometer in car interior with LCD thereon. LCD devices have monochrome display and passive driving circuits from earlier period to now with more than 64K levels and active driving circuits to promote applications of LCD devices.
Present LCD devices are using active driving circuits to drive liquid crystal. Thin film transistors (TFT) are commonly used to control alignment of liquid crystal in active LCD devices. Nevertheless, fabrication of TFT is complicated due to 4 to 5 photolithographic processes are required. Further, very complicated half-tone lithographic process or other trick lithographic process is introduced to reduce numeral lithographic processes, but LCD panel manufacture yield is lowered down.
One method for manufacturing active driving circuits is using thin film diode (TFD) as driving entity. Several advantages, such as simple structure as well as process, and higher yield, are shown within TFD, as shown in U.S. Pat. Nos. 5,926,236 and 6,008,872.
However, the current-voltage diagram is asymmetric in the above TFD, which encounters some issues in driving circuits. Moreover, remnant shade and inhomogeneous gray scale also happens here. Four TFDs are used to improve above issues, as shown in FIG. 1. A pixel cell 10 comprises a liquid crystal capacitor 20, 4 metal-insulation-metal (MIM) diodes 25, 26, 27, and 28, two selecting lines (also known as scanning lines) 12, 14, and a data line 60, wherein the liquid crystal capacitor 20 comprises a pair of pixel electrodes 21, 22, and liquid crystal molecule 23 between the pair of pixel electrodes 21, 22. This kind of structure is know as dual-select back-to-back TFD device, in which current-voltage diagram will be symmetric, as shown in FIG. 2. Related technique can be referred to U.S. Pat. No. 6,225,968, U.S. Pat. No. 6,243,062, JP publication 2002-043657, 2000-098429, 11-305267, TW publication 571169, 500947, and EPO 434627.
However, there is still issue here to be improved in dual-select back-to-back TFD device. As shown in FIG. 3, a portion of liquid crystal cell 10 in FIG. 2 is shown in schematic representations. A pixel electrode 21, four MIM diodes 25, 26, 27, and 28, and two selecting lines 12, 14, which all are on an active substrate, are shown in FIG. 3. Another pixel electrode 22, and data line 60 in FIG. 1 are on color filter substrate. When the active substrate and the color filter substrate are sealed, liquid crystal molecule will be injected into a space between the two substrates to form an LCD panel. Four TFDs 25, 26, 27, and 28 occupy a portion of pixel electrode 21, so that the pixel aperture is reduced.
Hence, a novel structure is necessary to increase pixel aperture for the MIM diode structure and also maintains simple process as well as high manufacture yield.